x^0+x^1+x^2+.+x^n
令s=x^0+x^1+x^2+.+x^n
xs=x^1+x^2+.+x^n+x^(n+1)
xs-s=x^(n+1)-x^0
s(x-1)=x^(n+1)-1
s=[x^(n+1)-1]/(x-1)